It is common for telephone sets in a business office to be coupled to the telephone network via a 25-pair distribution cable installed in the office. Modern telephone sets, however, are designed to be electrically connected up through the telephone cords and plugs which include a substantially lesser number of electrical conductors than those contained in the cable. For example, as compared to the 25 pairs of wire in the cable, some business telephone sets have a two-pair cord and a corresponding plug with four spring contacts, while others, have a three-pair cord terminating in a six-contact plug. Evidently, to try to electrically connect seriatim the four or six lead-in wires of such a set to the appropriate wires of the 25-pair cable would be unduly expensive and time consuming. This is particularly true when the telephone set is replaced from time to time by a new set which is either the same or different model. Therefore, it is usual for the coupling of the set to the cable to be accomplished by the use of devices known as adapters which are mounted in installed adapter blocks, and which, on the one hand, receive the plug of the set and, on the other hand, are coupled through their connector through a connector included in the adapter block to the conductors in the cable so as to permit, by simply inserting the plug of the set into the jack of the adapter, a connecting up of the telephone set to the cable and, thus, to the telephone network.
An adapter of such kind for coupling sets with two-pair or three-pair cords terminating in four-contact or six-contact plugs is disclosed on page 17 of the April 1980 issue of the Technical Digest, a publication of Western Electric Company, Incorporated. The disclosed adapter comprises a horizontal 50 pin connector, adapted to make with a connector in an adapter block, a hood for the connector and a jack received within the hood. The hood has an upside-down "U" shape in transverse cross section and terminates at its forward end in a vertical front opening of the same shape. The jack comprises a housing, a plug-receiving passage extending in such housing from its front towards its back, and spring contacts located at the rear of the passage.
For the purpose of receiving and maintaining the jack within the hood, the latter's upside-down "U" opening is bordered by two small vertical ridges projecting transversely inwards towards each other from the sidewalls of the hood, and the jack housing has two corresponding vertical grooves formed on its transversely opposite planar side walls. To fit the jack in the hood, the former is placed below the latter so as to vertically align the grooves in the jack housing with the ridges in the hood. The jack is then moved up with its grooves slidably receiving and sliding past such ridges until the jack reaches its final position inside the hood. Next, the hood is fastened at its forward end to the 50-pin connector by forcing locking tabs on the bottoms of the sidewalls of the hood past locking shoulders on the connector to snap mount the hood on the connector. When this is done, a flange on the connector projects outwardly beneath the bottom of the jack to prevent the jack from falling out of the hood.
When grooves are formed, as described, in the transversely opposite sides of the jack, the side walls of the jack housing must compensatingly be made transversely thicker in order for the housing to have the same mechanical strength as it would in the absence of such grooves. To put it another way, because of the previously used ridge-groove mode of securing the hack to the hood, the interior space within the jack housing cannot be transversely widened without sacrificing the mechanical integrity of the jack except by transversely widening the jack housing and, concomitantly, the transverse cross section of the good within which the jack is received. On the other hand, there are now models of key telephone sets which use four-pair cords and plugs with eight contacts so as to require eight contacts in the adapter jack, and to accommodate those eight jack contacts, the plug-receiving passage therein must commensurately be widened. If, however, in order to permit such passage's widening, the exterior of the jack's housing is transversely widened with the accompanying necessary transverse widening of the hood, then the adapter will not fit in the standard block which is used to mount the adapter in offices and which is already installed in many of them.
Accordingly, the adapter described in the mentioned Technical Digest article cannot conveniently be constructed so as to include a jack modified to have eight contacts and so as, at the same time, be dimensioned to still fit in a standard adapter block.